1. Field of the Invention
The present invention relates to a motor drive control circuit of a PWM (Pulse Width Modulation) control technique and also relates to a motor apparatus including such a motor drive control circuit.
2. Description of the Related Art
A motor apparatus using a conventional motor drive control circuit of a PWM control technique is shown in FIG. 4. A motor apparatus 101 shown in the figure includes a motor 102, a motor driver 107 for driving the motor 102, and a motor drive control circuit 106 for controlling the motor driver 107.
The motor 102 includes a rotor 109; coils LU, LV, LW of the U phase, V phase, and W phase for controlling the rotation of the rotor 109; Hall elements HU, HV, HW for detecting the position (phase) of the rotor 109; and a rotation speed counter 104 for detecting the rotation speed of the rotor 109. The motor driver 107 includes three output transistors TUU, TVU, TWU on the power source side and three output transistors TUL, TVL, TWL on the ground side. The motor drive control circuit 106 includes a current detection resistor 112 for converting a drive current of the motor 102 into a voltage; a peak hold circuit 114 for receiving this voltage and holding the peak voltage within the ON period of the below-described PWM signal; a rotation control amplifier 113 for inputting the peak voltage, the voltage limiting reference voltage of a reference voltage source 123, and a rotation speed control voltage of a signal input terminal SIG, and comparing the lower of the voltage limiting reference voltage and rotation speed control voltage with the peak voltage; a capacitor 122 for oscillation prevention that is connected to the output of the rotation control amplifier 113 and has a capacitance of, for example, about 0.01 μF; a Hall amplifier 116 for inputting the Hall signals of the Hall elements HU, HV, HW and outputting the amplified signals; a synthesis circuit 117 for inputting the output of the Hall amplifier 116, advancing each input by a constant phase (for example, 30°), conducting amplification at an amplification ratio corresponding to the output voltage of the rotation control amplifier 113, and outputting the amplified signals; a triangular wave generator 119 for generating and outputting a triangular wave; a PWM output comparator 118 for comparing the polarity discrimination signals UHL, VHL, WHL, which are the outputs of the synthesis circuit 117, as shown in FIG. 6, with the triangular wave and outputting PWM signals UPWM, VPWM, WPWM; and a motor-driver control circuit 120 for outputting to the motor driver 107 a control signal based on the PWM signals.
The detection output of the rotation speed counter 104 of the motor 102 is inputted to a motor control command unit (not shown in the figure) including a CPU. The CPU outputs a command signal (rotation speed control voltage) corresponding to the desired motor rotation speed to the signal input terminal SIG of the motor drive control circuit 106. If the detection output of the rotation speed counter 104 is less than the desired motor rotation speed, the CPU increases the rotation speed control voltage so as to obtain the desired motor rotation speed. In such a case, the rotation control amplifier 113 increases the output voltage thereof since the rotation speed control voltage becomes higher than the peak voltage. Therefore, in the synthesis circuit 117, the amplification ratio increases and the amplitude of the polarity discrimination signals UHL, VHL, WHL increases. Then, the PWM output comparator 118 generates the PWM signals UPWM, VPWM, WPWM having a duty ratio with a large ON period and outputs to the motor driver 107 the control signals based on PWM signals via the motor-driver control circuit 120. As a result, the drive current that is passed by the motor driver 107 to the coils LU, LV, LW of the U phase, V phase, W phase of the motor 102 increases and, therefore, the rotation speed of the motor 102 increases. Then, this drive current is converted into a voltage by the current detection resistor 112, and the peak voltage thereof is compared with the rotation speed control voltage of the signal input terminal SIG, as described above. This loop operation is repeated and the peak voltage of the detection voltage is thereby matched with the rotation speed control voltage and stabilized.
Here, when an overload is applied to the motor 102 (referred to below as an abnormality), for example, when paper is jammed in the case where the motor 102 is used as a paper feed actuator of a copier, the detected rotation speed by the rotation speed counter 104 decreases. Therefore, the CPU increases the rotation speed control voltage according to the detected rotation speed in order to increase the rotation speed of the motor 102. However, because the rotation speed of the motor 102 does not increase, this rotation speed control voltage rises too much. If the voltage limiting reference voltage of the reference voltage source 123 is exceeded, the voltage limiting reference voltage becomes lower and is, therefore, compared with the peak voltage. Thus, when the rotation speed control voltage rises too much, the excess increase of the drive current of the motor driver 107 flowing to the coils LU, LV, LW of the U phase, V phase, W phase of the motor 102 is prevented and damage to the elements is prevented (for example, see Japanese Patent Application Laid-open No. 2003-111481).
As described above, when the rotation speed control voltage from the CPU rises too much and exceeds the voltage limiting reference voltage of the reference voltage source 123, the voltage limiting reference voltage, rather than the rotation speed control voltage, is compared with the peak voltage in the rotation control amplifier 113. However, since the capacitor 122 for oscillation prevention that has a capacitance of about 0.01 μF is connected to the output of the rotation control amplifier 113, the output thereof is delayed, a time is required for the output to be reflected in the synthesis circuit 117, and a time is also required for the output to be reflected in the output of the PWM output comparator 118 and then in the rotation speed of the motor 102, which is the final output.
FIG. 5 shows a voltage waveform detected by the current detection resistor 112 in the case of an abnormality. The voltage E in the figure is a voltage limiting reference voltage of the reference voltage source 123, and below this voltage is a region in which the elements defining the motor driver 107 are not damaged, that is, an element safe operation region. The amplitudes of polarity discrimination signals UHL, VHL, WHL inputted to the PWM output comparator 118 include an extra amount due to the above-described delay of the output of the rotation control amplifier 113, the PWM signals UPWM, VPWM, WPWM containing an extra ON time period in the duty ratio are outputted from the PWM comparator 118, and the motor driver 107 passes an extra drive current. Thus, in the conventional motor apparatus, a time period (for example, a time period shown by points A-B in FIG. 5) occurs in which the motor drive 107 operates above the element safe operation region. Furthermore, the power consumed in this period is wasted power.
As a countermeasure, an extra voltage that is due to the delay time caused by the capacitor 122 for oscillation prevention can be estimated and the voltage limiting reference voltage of the reference voltage source 123 can be set accordingly lower. However, because a load at the time of abnormality is not constant, the peak value of the voltage detected by the current detection resistor 112 varies, for example, as voltage C or voltage D shown in FIG. 5. Therefore, this countermeasure is actually difficult to use.